Decades of research have shown that complement activation contributes to tissue injury following myocardial ischemia and reperfusion (MI/R). The long term objectives of our research are to understand the tissue injurious aspect of complement activation in myocardial ischemia and develop therapeutic strategies to regulate complement activation following MI/R. The precise mechanisms of complement-induced injury are unknown, but include recruitment of inflammatory cells to the site, production of anaphylatoxins that mediate tissue damage, and possible tissue damage due to excessive membrane attack complex (MAC) deposition leading to cell necrosis. Recent clinical trials using anti-C5 therapy (pexelizumab), which inhibits the formation of both C5a anaphylatoxin and MAC, were unsuccessful in preventing the size of myocardial infarctions, despite the successful use of anti-C5 therapies in preventing injury in animal models. Recent studies have shown that formation of sublytic amounts of MAC activates cell cycle and survival signaling pathways. Preliminary data in this fellowship demonstrate that C6 deficiency increases myocardial injury compared C6 sufficient animals. In light of these reports and preliminary data, we hypothesize that low concentrations of the MAC may act as a protective agent in MI/R injury. To test this hypothesis, we will investigate these specific aims: 1. Establish the roles of C5a and C5b-9 in cardiomyocyte cell death following oxidative stress. 2. Investigate the roles of C5a and C5b-9 in mediating myocardial ischemia/reperfusion injury in vivo. These aims will be addressed by subjecting adult rat ventricular myocytes (ARVM) to anoxia and reoxygenation in the presence of rat serum from wild type animals or animals lacking C6 and are therefore unable to form the MAC. To address the role of C5a, we will test a pharmacological inhibitor of the C5a receptor on ARVM damage. We will also use in vivo models of MI/R in rats and mice to determine contributions of C5a and C5b-9 in tissue injury and inflammation. Rats and mice deficient in C6, as well as pharmacologic inhibition of the C5a receptor will be investigated. These studies will demonstrate the importance of C5 cleavage products, C5a and C5b-9, in mediating tissue injury and inflammation following MI/R. The results from this study will provide a rational plan for the inhibition of biologically active complement molecules formed following a heart attack. Drugs currently available may be better used to protect the heart from damage following a heart attack. [unreadable] [unreadable] [unreadable] [unreadable]